The speed range of an electrical machine is determined by many factors, including but not limited to, pole number, turns per phase and supply voltage. Brushless electronically commutated synchronous motors include permanent magnet brushless dc and brushless ac, hybrid stepping motors, synchronous reluctance, flux switching and switched reluctance motors. In such motors it is particularly difficult to rotate smoothly in the lower part of the speed range. This is especially true when driving a low inertia load in which small changes in torque of the motor or the load can cause very large changes in motor speed. Rotation at low speed is particularly difficult because the inherent torque ripple of the machines interacts with controller response and the interaction of the load dynamics to create an extremely complex system which is difficult to tune and keep stable under all conditions.
An electronically commutated motor relies on knowledge of the position of the rotor to correctly excite particular phase windings in the motor to deliver torque of the required magnitude and direction. This rotor position information can be obtained from a shaft encoder. However to reduce cost the encoder resolution will often be very low. At low speeds, changes in state of the encoder may only occur six times per electrical cycle. If a motor is turning slowly there is a considerable time between position updates making a speed control system very difficult to implement.
Methods to detect the rotational EMF in the motor windings have now become common but they deteriorate at low speeds due to the decrease in the value of the rotational EMF reducing the signal to noise ratio to a level where parameter variations in the EMF estimation algorithm can produce significant position feedback errors.
Smooth rotation at low speed is therefore a problem for electronically commutated synchronous motors with and without sensors on the rotor. In most cases where this is required, the solution has been to fit a high resolution encoder. However this is expensive and not appropriate for many applications.